Micro-encapsulation of components and incorporation of such into energetic formulations

ABSTRACT

Method for controlling the release of a component in an energetic material formulation comprising encapsulating at least one component in the energetic material formulation, said component selected from the group consisting of a stabilizer, a plasticizer, a burn rate modifier, and combinations thereof, in a suitable encapsulation material; and admixing the encapsulated selected components into the energetic material formulation.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 60/933,742, filed on Jun. 8, 2007.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a method for encapsulation of selected components prior to incorporation into, e.g., energetic formulations, and further relates to formulations comprising encapsulated components.

BACKGROUND OF THE INVENTION

Energetic compositions are a heterogeneous composite of oxidizer, fuel, binder and other additives such as plasticizers, burning rate modifiers and curing agents. The heterogeneous nature of the material lends itself to lot-to-lot production variability as well as a wide variety of aging processes. Both production variability and aging have been noted as possible contributors to propellant performance problems. Moderate environmental conditions may have little impact on the service life of energetic formulations, however temperature extremes, temperature cycling, and drastic humidity changes may have a more significant impact in a short period of time. Aging studies, spectroscopy, and mechanical testing have focused on the polymer binder, typically hydroxyl-terminated polybutadiene (HTPB), used in solid composite propellants.

Polymer binders provide both mechanical integrity and adhesion for the composite. Other composite ingredients have received less attention, such ingredients including, e.g., plasticizers, stabilizers or burn rate modifiers. Propellant research indicates that the concentration of certain components such as the polymer binder and oxidizer change very little with time. However, plasticizer evaporation and diffusion is quite common and may possibly contribute to the mechanical properties and physical aging of the composite, especially at extreme storage conditions. A method for controlled delivery of all, or a portion of other composite ingredients could lead to a more uniform concentration profile of such ingredients within the composite. This could lead to more uniform mechanical properties and reduced physical aging, in turn leading to increased reliability, performance and safety.

Encapsulation has been used extensively in a number of areas, including pharmaceuticals and cosmetic compositions. See, for example, Berchane et al., entitled “Effect of mean diameter and polydispersity of PLG microspheres on drug release: Experiment and Theory”, International Journal of Pharmaceutics, 337 (2007) 118-126, and Berchane et al., “About mean diameter and size distributions of poly(lactide-co-glycolide) (PLG) microspheres”, Journal of Microencapsulation, August 2006, 23(5) 539-552, both of which are incorporated herein by reference. However, encapsulated materials, and in particular timed-release delivery systems, are not believed to have been used in energetic compositions to date. The incorporation of timed-release materials in such compositions presents a number of challenges. For example, the propellant must remain stable over a long period of time so as to prevent uncontrolled chemical reaction with additives, resulting in self-ignition. The encapsulated component further muct be compatible with a complex and reactive mixture and may not interfere with intended energy release. A need exists therefore, for energetic compositions comprising timed-release materials, and methods for controlling the release of selected components in energetic compositions.

SUMMARY OF THE INVENTION

The present invention utilizes encapsulated components suitable for use in an energetic composition that comprise a timed release delivery system with desired release kinetics. The invention utilizes polymeric microspheres of specific mean diameter, size distribution, polymer molecular weight. The timed release patterns may produce a constant release for a wide range of durations that cover from minutes to months, or alternatively a pulsed release. Applicants believe that encapsulation can be used to protect and ensure timed release of reactive additives, such as stabilizers, which in turn prolongs the lifetime of the propellant. In addition, encapsulation of additives may prevent consumption of additives during manufacturing.

The following describe some non-limiting embodiments of the present invention.

According to one embodiment of the present invention, an energetic material formulation comprising at least one encapsulated component selected from the group consisting of plasticizers, stabilizers, burn rate modifiers and combinations thereof is provided.

According to another embodiment of the present invention a method for controlling the release of a component in an energetic material formulation is provided, comprising encapsulating at least one component in the energetic material formulation, said component selected from the group consisting of a stabilizer, a plasticizer, a burn rate modifier, and combinations thereof, in a suitable encapsulation material; and admixing the encapsulated selected components into the energetic material formulation.

DETAILED DESCRIPTION

In all embodiments of the present invention, all percentages are by weight of the total composition, unless specifically stated otherwise. All ratios are weight ratios, unless specifically stated otherwise. All ranges are inclusive and combinable. The number of significant digits conveys neither a limitation on the indicated amounts nor on the accuracy of the measurements. All numerical amounts are understood to be modified by the word “about” unless otherwise specifically indicated. All documents cited in the Detailed Description of the Invention are, in relevant part, incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention. To the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern.

“Encapsulation,” as used herein, means that portions of one or more components of the composition are substantially enclosed in a suitable encapsulation material, such that the encapsulation material is adhered to a particular component, wherein the component may be a plasticizer, a stabilizer, a burn-rate modifier, or other component of an energetic formulation.

“Suitable encapsulation material,” or “encapsulant,” as used herein, means a material that is sufficiently robust to withstand formulation and manufacturing conditions of energetic compositions; is compatible with the formulation, does not adversely impact propellant performance, and does not adversely impact other aspects of intended propellant functioning. In addition, a suitable encapsulation material adheres to the component to be encapsulated (as opposed to other materials in the composition, such as binders, which may surround a material, but which does not constitute encapsulation as envisioned herein). Adhesion of the encapsulant may occur through covalent chemical bonding or through non-covalent interactions (e.g., ionic, van der Waals, dipole-dipole etc.).

“Microencapsulated,” as used herein, means that the average diameter of the encapsulated component is from about 1 μm to about 1000 μm. If the encapsulated component is oblong or asymetrical, then the average diameter is measured across that part of the component having the greatest length.

“Energetic material formulation,” as used herein, means a formulation or composition having a high amount of stored, releasable chemical energy, and includes but is not limited to explosives, pyrotechnic compositions, propellants (e.g. smokeless gunpowders and rocket fuels), and fuels (e.g. diesel fuel and gasoline).

The present invention relates to an energetic composition comprising an encapsulated component capable of timed-release, and further to a method for controlling the release of a selected component in an energetic material formulation. By controlling the time of release of selected components, the properties of the energetic material formulation may be improved, for example, longer shelf life (i.e., a formulation comprising encapsulated components functions in a substantially similar manner for a longer period of time when compared to a similar composition that does not contain the encapsulated components), greater release of energy, improved batch-to-batch consistency, increased stability under extreme environmental conditions, such as heat, humidity, changes in pressure, exposure to mechanical shocks, etc., and the amount of components consumed during manufacture. The encapsulated components of the present invention may be included in compositions such as composite solid rocket motor propellants, single-, double-, and triple-base propellant compositions and other energetic material formulations where stability of the encapsulated components are essential to maintaining and/or extending the shelf-life of the composition. Accordingly, the present invention further is aimed at controlling component mobility, size of the encapsulated component, and chemical degradation thereof.

The energetic composition may be an explosive, a pyrotechnic composition, a propellant, a smokeless powder, and/or a fuel. In one embodiment, the energetic composition is a smokeless powder comprising a single-base, double-base and/or triple-base powder.

The stability and lifetime of double-base propellants is of significant interest since added stabilizers are consumed over time, leaving the propellant prone to uncontrolled chemical reactions, high internal pressure regions, and even auto-ignition. Double-base propellants, based on nitrocellulose and nitroglycerin, undergo degradation even at ambient storage conditions. It is believed that the stability and lifetime of double-base propellants, such as 2-nitrodipenylamine (2NDPA) and N-methyl-p-nitroaniline (MNA) and the like, may be extended by encapsulating or pre-encapsulating the plasticizers, stabilizers and other components used within the formulation, thus allowing for a controlled, gradual release of the plasticizer and/or stabilizer rather than uncontrolled degradation. The controlled release may be from within cellulose or other encapsulation polymers suitable for energetic material formulations. In one embodiment, the controlled release occurs over a time period of from about 1 minute to about 60 minutes, and alternatively from about 1 minute to about 30 minutes, and alternatively from about 1 minute to about 5 minutes. In an alternative embodiment, the timed release occurs over a time period of from about 1 day to about 30 days. In an alternative embodiment, the timed release occurs over a period of from about 1 month to about 12 months, and alternatively from about 1 month to about 6 months. In one embodiment, the timed release is a pulsed time release, wherein the encapsulated material is released intermittently as opposed to continuously.

Encapsulation may occur as exemplified herein, in addition to other methods of microencapsulation which would be understood by one of skill in the art, non-limiting examples of which include phase separation, solvent evaporation, solvent extraction, in-situ polymerization, interfacial polymerization, atomization using spray drying and chilling, use of a rotating disk, spray coating with a fluidized bed, spray drying and co-extrusion, and combinations thereof.

In one embodiment, the component is microencapsulated, and the encapsulated product has an average diameter of from about 1 μm to about 1000 μm, alternatively from about 1 μm to about 120 μm, alternatively from about 1 μm to about 50 μm, and alternatively from about 1 μm to about 25 μm. Non-limiting examples of suitable encapsulation materials include polystyrene, methacrylates, polyamides, nylons, polyureas, polyurethanes, gelatins, polyesters, polycarbonates, modified polystyrenes, and ethylcellulose degradable polymer matrices. In one embodiment, the encapsulation material is poly(lactide-co-glycolide) (PLG), poly(glycidylmethacrylate)(PGMA), polystyrene, or combinations thereof. Suitable encapsulation materials may have a molecular weight of from about 5 kDa to about to about 250 kDa, alternatively from about 200 kDa to about 250 kDa, alternatively from about 50 kDa to about 75 kDa, alternatively from about 10 kDa to about 50 kDa and alternatively from about 10 kDa to about 25 kDa.

Non-limiting examples of plasticizers that may be encapsulated include dioctyl adipate (DOA), dioctyl azelate (DOZ), isodecyl dipenylphosphate (IDP), dibutyl phthalate, dinitrotoluene and combinations thereof.

Non-limiting examples of stabilizers that may be encapsulated in accordance with the invention include 2-nitrodiphenylamine (or tertiary butylcatechol), p-nitromethylaniline (or N-methyl-p-nitroaniline), diphenylamine, nitrodiphenylamine, methyl centralite, ethyl centralite and combinations thereof.

Non-limiting examples of suitable burn rate modifiers include alkali metals, an alkaline earth or a transition metal salt of tetrazoles or triazoles; an alkali metal or alkaline earth nitrate or nitrite; triaminoguanidine nitrate; dicyandiamide, and alkali and alkaline earth metal salts of dicyandiamide; alkali and alkaline earth borohydrides; and mixtures thereof. The composition may comprise from about 0.01% to about 10% of a burn rate modifier.

Non-limiting examples of suitable binder materials include cellulose based binders such as cellulose acetate butyrate (CAB), polycarbonates, polyurethanes, polyesters, polyethers, polysuccinates, thermoplastic rubbers, polybutadiene, hydroxy-terminated polybutadiene polystyrene, polybutadiene acrylonitrile (PBAN), and mixtures thereof.

The following are some non-limiting examples of the present invention. Although described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention, except as and to the extent that they are included in the accompanying claims.

EXAMPLES Example 1

The following describes one manner of synthesizing a micro-encapsulated stabilizer.

Poly(lactide-co-glycolide) (PLG) microspheres containing 2-nitrodiphenylamine (2-NDPA) were synthesized as follows. PLG microspheres were prepared using an oil-in-water (o/w) emulsion method followed by solvent extraction. Approximately 8 g of poly(vinyl alcohol) (PVA) (87-89% hydrolyzed, MW of 13-23 kDa) was added to 92 mL of deionized water in a 400 mL Pyrex beaker. The PVA was dissolved with stirring and gentle heating. After the PVA dissolved, the solution was allowed to cool to room temperature. Approximately 1 mL of 0.1% antifoam agent was added to the PVA solution to prevent excessive foaming later in the preparation. Approximately 0.125 g of 2-nitrodiphenylamine (2-NDPA) (20% theoretical loading based on PLG mass) was weighed together with 0.50 g of PLG (50:50 lactide:glycolide, MW 40-75 kDa). 2-NDPA and PLG were dissolved together in 4.5 mL dichloromethane (DCM). The PVA solution was stirred at the desired speed for 5 min in a 400 ml Pyrex beaker with a Caframo™ ultra high torque stirrer. The 2-NDPA/PLG solution in DCM was slowly added to the beaker and stirring was continued for 60 min. The resulting emulsion was poured into 1 L of deionized water in a 2 L Pyrex beaker. The stirring speed was increased to 1200 rpm for 90 minutes. The PLG microspheres containing 2-NDPA were then collected by vacuum filtration using a fine porosity filter paper (1-5 μm particle retention) to prevent loss of microspheres.

Example 2

Example 2 describes another manner of synthesizing a micro-encapsulated stabilizer.

Polystyrene (PS) microspheres containing 2-nitrodiphenylamine (2-NDPA) were synthesized as follow. PS microspheres were prepared using an oil-in-water (o/w) emulsion method followed by solvent extraction. Approximately 8 g of poly(vinyl alcohol) (PVA) (87-89% hydrolyzed, MW of 13-23 kDa) was added to 92 mL of deionized water in a 400 mL Pyrex beaker. The PVA was dissolved with stirring and gentle heating. After the PVA dissolved, the solution was allowed to cool to room temperature. Approximately 1 mL of 0.1% antifoam agent was added to the PVA solution to prevent excessive foaming later in the preparation. Approximately 0.125 g of 2-nitrodiphenylamine (2-NDPA) (20% theoretical loading based on PLG mass) was weighed along with 0.50 g of PS (MW 230 kDa). 2-NDPA and PS were dissolved together in 4.5 mL dichloromethane (DCM). The PVA solution was stirred at the desired stirring speed for 5 min in a 400 ml Pyrex beaker with a Caframo™ ultra high torque stirrer. The 2-NDPA/PS solution in DCM was slowly added to the beaker and stirring was continued for 60 min. The resulting emulsion was poured into 1 L of deionized water in a 2 L Pyrex beaker. The stirring speed was increased to 1200 rpm for 90 minutes. The PS microspheres containing 2-NDPA were then collected by vacuum filtration using a fine porosity filter paper (1-5 μm particle retention) to prevent loss of microspheres.

Example 3

Example 3 describes one non-limiting example of a single-base propellant formulation of the present invention.

Single Base Material Wt % Nitrocellulose <98 Dinitrotoluene  0-16 Microencapsulated 0.5-3.0 Diphenylamine Potassium Nitrate 1.0-1.5 Potassium Nitrate   0-1.5 Potassium Sulfate 0-1 Graphite 1.5 mx Ethyl Centralite 0-6 Methyl Centralite 0-2

Example 4

Example 4 describes one non-limiting embodiment of a double-base propellant formulation of the present invention.

Material M2 M5 M8 M21 N5 MDM Nitrocellulose 77.45 81.95 52.15 53.0 50.0 48.6 (13.25% N) (12.6% N) Nitroglycerin 19.50 15.00 43.00 31.0 34.9 27.0 Potassium Nitrate 2.15 2.15 1.25 — — — Ethyl Centralite 0.6 0.6 0.6 2.0 —  1.1 Graphite 0.3 0.3 — — — — Triacetin — — — 11.0 — 18.7 Lead Salicylate — — — 2.5 — — Lead Stearate — — — 0.5 —  4.6 Carbon Black — — — 0.03 — — Diethyl Phthalate — — 3.0 — 10.5 — Microencapsulated — — — —  2.0 — 2-nitrodiphenyl amine (2NDPA) Lead salts — — — —  2.4 — Candelilla wax — — — —  0.2 — 

1. An energetic material formulation comprising at least one encapsulated component selected from the group consisting of plasticizers, stabilizers, burn rate modifiers and combinations thereof.
 2. The energetic material formulation of claim 1 wherein the formulation is an explosive, a pyrotechnic composition, a propellant, a smokeless powder, or a fuel.
 3. The energetic material formulation of claim 2 wherein the formulation is a smokeless powder comprising a single-base powder, a double-base powder or a triple-base powder.
 4. The energetic material formulation of claim 1 wherein said component is microencapsulated.
 5. The energetic material formulation of claim 4 wherein the microencapsulated component has an average diameter of from about 1 μm to about 120 μm.
 6. The energetic material formulation of claim 1 wherein the component is encapsulated with a material selected from the group consisting of methacrylates, polyamides, nylons, polyureas, polyurethanes, gelatins, polyesters, polycarbonates, modified polystyrenes, ethylcellulose degradable polymer matrices, and combinations thereof.
 7. The energetic material formulation of claim 1 wherein the component is encapsulated with a material selected from the group consisting of poly(lactide-co-glycolide), poly(glycidylmethacrylate), polystyrene, and combinations thereof.
 8. The energetic material formulation of claim 1 wherein the stabilizer is selected from the group consisting of 2-nitrodiphenylamine, p-nitromethylaniline, diphenylamine, nitrodiphenylamine, methyl centralite, ethyl centralite and combinations thereof.
 9. The energetic material formulation of claim 1 wherein the plasticizer is selected from the group consisting of dioctyl adipate, dioctyl azelate, isodecyl dipenylphosphate and combinations thereof.
 10. A method for controlling the release of a component in an energetic material formulation comprising: encapsulating at least one component in the energetic material formulation, said component selected from the group consisting of a stabilizer, a plasticizer, a burn rate modifier, and combinations thereof, in a suitable encapsulation material; and admixing the encapsulated selected components into the energetic material formulation.
 11. The method of claim 10 wherein said suitable encapsulation material is selected from the group consisting of methacrylates, polyamides, nylons, polyureas, polyurethanes, gelatins, polyesters, polycarbonates, modified polystyrenes, ethylcellulose degradable polymer matrices, and combinations thereof.
 12. The method of claim 10 wherein said suitable encapsulation material is selected from the group consisting of poly(lactide-co-glycolide), poly(glycidylmethacrylate), polystyrene, and combinations thereof.
 13. The method of claim 10 wherein said stabilizer is selected from the group consisting of 2-nitrodiphenylamine, p-nitromethylaniline, diphenylamine, nitrodiphenylamine, methyl centralite, ethyl centralite and combinations thereof.
 14. The method of claim 10 wherein said plasticizer is selected from the group consisting of dioctyl adipate, dioctyl azelate, isodecyl dipenylphosphate and combinations thereof.
 15. The method of claim 10 wherein said component is microencapsulated.
 16. The method of claim 15 wherein the microencapsulated component has an average diameter of from about from about 1 μm to about 120 μm.
 17. The method of claim 1 wherein the component is substantially continuously released into the formulation.
 18. The method of claim 1 wherein the component is intermittently released into the composition. 